The present invention relates to the general field of systems for injecting fuel into a combustion chamber in a gas turbine engine. The invention relates more particularly to an injection system that includes in particular an aerodynamic fuel injector having multipoint fuel feed.
In conventional manner, the combustion chamber of a gas turbine engine is provided with a plurality of injection systems enabling it to be fed with fuel and air at all of the operating speeds of the engine. The injection systems comprise in particular fuel injectors and air admission means downstream from the injectors. There are two main categories of fuel injector: there are xe2x80x9caeromechanicalxe2x80x9d injectors designed to deliver two fuel flows depending on engine speed; and there are xe2x80x9caerodynamicxe2x80x9d injectors which have a single fuel circuit for use at all engine speeds. In addition, certain xe2x80x9caerodynamicxe2x80x9d injectors present air feed channels at the end or xe2x80x9cnosexe2x80x9d of the injector in order to deliver an air/fuel mixture directly. The present invention relates more particularly to injection systems including socalled xe2x80x9caerodynamicxe2x80x9d injectors that also inject air.
The air admission means known in the prior art generally comprise primary and secondary swirlers which deliver a swirling flow of air at the outlet of the fuel injector. A Venturi separating the two swirlers serves to accelerate the flow of air from the primary swirler, and a bowl mounted downstream from the secondary swirler enables the injector to be mounted on the end wall of the combustion chamber while preventing the flame due to combustion of the air/fuel mixture from returning towards the injector.
That type of injection system presents drawbacks. In particular, the air/fuel mixture delivered at the outlet of the injector is generally non-uniform, thus increasing pollution emissions from the engine. The speed at which fuel outlet from the injector flows is also insufficient, in particular at low flow rates, thus running the risk of coking on the nose of the injector and giving rise to non-uniformities in the air/fuel mixture. A low fuel flow speed also has the drawback of increasing the risk of the flame due to combustion of the air/fuel mixture coming back as far as the end of the injector which is prejudicial to proper operation of the gas turbine. In addition, after repeated engine-lighting operations using that type of injection system, traces of coking are found to appear between the injector body and the bowl.
The present invention thus seeks to mitigate such drawbacks by proposing an injection system in which the fuel injector enables a more uniform mixture of air and fuel to be obtained and also provides a greater flow speed for the fuel at its outlet.
To this end, the invention provides a system for injecting an air/fuel mixture into a combustion chamber of a turbomachine, the system having an injector comprising: an axial internal volume opening out at one end via an axial outlet for the air/fuel mixture; a first fuel feed stage having a plurality of first fuel feed orifices which open out into the internal volume, which are distributed around an axis of the injector, and which are connected by fuel feed channels to an inlet for admitting fuel into the injector; and at least one air feed channel which opens out into the internal volume and which is connected to an inlet for admitting air into the injector; wherein the injector further comprises at least one second fuel feed stage with a plurality of second fuel feed orifices which open out into the internal volume, which are distributed around the axis of the injector, and which are connected to said inlet for admitting fuel into the injector via fuel feed channels which coincide at least in part with the fuel feed channels of said first stage.
As a result, the second fuel feed stage enables the number of fuel feed points into the inside volume of the injector around the axis thereof to be increased. This has the result of improving the uniformity of air/fuel mixing.
The first and second fuel feed orifices, and also the air feed channel(s) open out into two coaxial passages formed in the internal volume. In an advantageous disposition of the invention, the passage into which the fuel feed orifices open out presents a section that tapers in the fuel flow direction. This characteristic makes it possible to increase the fuel flow speed so as to improve the ability of the injector to withstand coking, and so as to make the sheet of fuel more uniform, particularly at low fuel flow rates.
According to another advantageous disposition of the invention, the second fuel feed orifices are axially offset relative to the first fuel feed orifices. Under such circumstances, the second fuel feed orifices are preferably in angular positions around the axis of the injector that are offset relative to the positions of the first fuel feed orifices. These advantageous dispositions favor distributing fuel around the axis of the injector and thus encourage uniform air/fuel mixing.
According to yet another advantageous disposition of the invention, the fuel feed channels have terminal portions adjacent to the first and second fuel feed orifices that are oriented substantially tangentially relative to the wall of the internal volume. This characteristic makes it possible to set the fuel into rotation in the internal volume, thereby improving the flow speed and the uniformity of air/fuel mixing.
The injector preferably includes a rear part having the air feed channel(s) formed therein, at least one ring in which the first and second fuel feed stages are formed that is inserted in a housing formed at the downstream end of the rear part, and a front part which connects to the rear part, the ring being prevented from moving axially between the rear part and the front part of the injector.
According to another advantageous characteristic of the invention, each fuel feed stage has four fuel feed orifices distributed regularly around the axis of the injector.
The system of the invention further comprises a bushing surrounding at least a portion of the injector, a bowl forming a diverging portion for mounting the injection system on an end wall of a combustion chamber, at least one air swirler interposed between the bushing and the bowl, and a Venturi formed between the bowl and the portion of the injector surrounded by the bushing. An air passage is preferably provided between the bushing and the portion of the injector that is surrounded by the bushing so as to prevent coke forming at the nose of the injector, and air flow holes are formed through the wall of the bowl forming a diverging portion.